Recently several papers on the efficient generation of second harmonic in single-mode optical fibers1,2 have appeared. The common feature of these experimental results are as follows: (1) Single-mode silica fibers codoped with oxides of germanium and phosphorous and cutoff wavelength in 0.5-1.3 µm were used. (2) The second harmonic generation was observed after fibers were exposed to mode-locked Q-switched pulses (1.06 µm) from a Nd:YAG laser for several hours. The peak power coupled was as high as 70 kW. During this long exposure the second harmonic was observed to build up from the noise level by several orders of magnitude with conversion efficiency approaching 5%. In another experiment the long exposure or preparation time was reduced to several minutes2 by subjecting the fiber simultaneously to the fundamental and its second harmonic generated by a KTP crystal.

© 1988 Optical Society of America

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